1. Field of the Invention
The present invention relates to an apparatus and method of generating of an encryption key used in coding (encryption) and decoding (decryption) of digital data with plural types of hierarchical scalabilities and, more particularly, to an apparatus and method of automatically generating partial encryption keys corresponding to respective data units in hierarchies in each scalability.
2. Related Background Art
In recent years, the spread of information and communications services through networks has also increased services to transmit data to unspecified masses, e.g., a delivery service of digital contents such as images (including one-frame data of a moving picture). In conjunction therewith, there is a demand for highly advanced functionality in protection technology of digital data.
In general, a coded digital image or the like is decoded in a quality (distortion, resolution, color representation, or the like) determined in a coding process. With diversification of communication channels, diversification of communication terminals, and diversification of delivery services, there is a demand for capability of decoding the image in a quality different from the quality determined in the coding process, by decoding a certain part of a codestream, i.e., scalability. For meeting this demand for scalability, for example, JPEG2000 (Joint Photographic Experts Group 2000) being the international standard of image compression provides hierarchized scalabilities with scales such as resolution. In the protection technology of hierarchically protecting data in different qualities, it is common practice to perform encryption using individual partial keys for respective data units located in respective hierarchies in each of scalabilities.
The known protection technologies of digital data as described above include, for example, those of Japanese Patent Application Laid-open No. 2004-312740 (Patent Document 1), Japanese Patent Application Laid-open No. 2003-204321 (Patent Document 2), Y. Wu, D. Ma, and R. H. Deng, “Progressive protection of JPEG 2000 condestreams.” In Proc. IEEE ICIP, pp. 3447-3450, 2004 (Non-patent Document 1), M. Fuhiyoshi, S, Imaizumi, and H. Kiya, “Encryption of composite multimedia contents for access control,” IEICE Trans. Fundamentals, Vol. E90-A, No. 3, pp. 590-596, March 2007 (Non-patent Document 2), and Shoko Imaizumi, Masaaki Fujiyoshi, Yoshito Abe, and Hitoshi Kiya, “Hierarchical encryption method of JPEG2000 for coded images with resistance to collusion attacks,” IEICE SIP symposium, 2006 (Non-patent Document 3).
Non-patent Document 1 discloses the technology of generating partial keys corresponding to data units in lower hierarchies from one master key by applying a one-way hash function to digital data with hierarchical scalabilities. Non-patent Document 2 discloses the technology independent of an order of streaming data, which is a problem of Non-patent Document 1. Furthermore, Non-patent Document 3 cited above discloses the technology of improving the resistance to collusion attacks, which is a problem of Non-patent Document 1.
A collusion attack is such an act that plural types of encryption keys corresponding to different hierarchical levels in respective scalabilities are shared among a plurality of users, so as to implement reproduction of the image in a quality higher than a preliminarily authorized quality.